The present invention concerns soft magnetic alloy powder for electromagnetic and magnetic shield, and shielding members containing the alloy powder.
Along the rapid progress of recent technologies concerning electronic appliances such as personal computers, word-processors, facsimiles and portable telephones, various problems caused by electromagnetic waves have been drawing attention. For example, electromagnetic wave generated by the parts such as IC and LSI causes erroneous working of appliances or become obstacles to wireless communication. Also, magnetic field generated by cables for power supply, electric motors or superconductive magnets influence on measuring instruments, sensors and electric/electronic circuits. As the means for solving these problems various magnetic shield and electromagnetic shield are used, and materials for the shield have been developed.
For the purpose of shielding electromagnetic wave and magnetic field (hereinafter represented by electromagnetic waves and referred to simply as "shield" or "shielding") use of plates of a metal having high magnetic permeability and saturation magnetic flux density such as permalloy is effective. However, if it is intended to cover and separate the space, in which electronic circuits or computers are installed, and from which influence of the electromagnetic wave must be excluded, with rolled plates of permalloy or the like, a shielding member itself will be considerably heavy. Also, it will be sometimes necessary to cut, bend or even weld the plates. This is laborious work and increases costs for shielding.
In general, alloys of high magnetic permeability and saturation magnetic flux density are expensive. The above mentioned processing causes distortion in the metal structure at the processed parts and results in lowering of shield properties. Because of this reason, plates of magnetic alloys are not so often used as the material for magnetic shield.
Then, as the material for magnetic shield which is easy to process and used for covering, there has been used paints prepared by suspending flat particles of the magnetic alloy having high magnetic permeability and saturation magnetic flux density in an appropriate vehicle, painted materials prepared by painting said paint on a suitable flexible support, or sheets prepared by dispersing the flat particles in a matrix of an organic material.
Shielding members of this kind using soft magnetic alloy powder disclosed to date are as described below. First examples are magnetic shielding paint prepared by crushing a soft magnetic amorphous alloy and mixing the resulting flat particles with a high polymer substance (Japanese Patent Disclosure No. 59-201493), and paint for magnetic shield prepared by mixing flat particles of high magnetic permeability alloy in a high polymer substance (Japanese Patent Disclosure No. 58-59268). In these paints the soft magnetic material powder should be of an aspect ratio as large as possible, or flat in the shape. Shield will be effective if the particles are placed in such direction that flat faces are rectangular to the direction of electromagnetic wave. This will ensure insulating property of the shielding material at the same time of the shielding effect.
Another paint for shield published comprises flat, but irregular particles of Fe--Ni alloys, Fe--Ni--Co alloys, Fe--Si--Al alloys and Fe--Ni--Mo alloys, or permalloy, molybdenum permalloy and sendust, suspended in high molecular substance matrix (Japanese Patent Publication No. 62-58631). The publication contains magnetic shield film using permalloy. Further proposal was to use coating films of magnetic iron containing one of Cr: 0.5-20 wt. %, Si: 0.5-9 wt. % and Al: 0.5-15 wt. % for shielding protection.
Also disclosed were a soft magnetic powder for magnetic shield, which is a flat, soft magnetic particles having an alloy composition falling in the pentagonal defined by the following points A, B, C, D and E in a ternary system of Fe, Si and Cr:
A: Fe.sub.78 Si.sub.22 Cr.sub.0 PA1 B: Fe.sub.70 Si.sub.30 Cr.sub.0 PA1 C: Fe.sub.60 Si.sub.30 Cr1.sub.0 PA1 D: Fe.sub.63 Si.sub.18 Cr.sub.19 PA1 E: Fe.sub.76 Si.sub.18 Cr.sub.6
a method of producing the powder and magnetic shielding members comprising the powder and a binder.(Japanese Patent Disclosure No. 3-295206)
Still other disclosure concerns a electromagnetic wave interference suppresser made by coating one or both surfaces of an electroconductive support with insulating soft magnetic layer which comprises flat fine powder of a soft magnetic material such as Fe--Al--Si alloy and an organic binder. (Japanese Patent Disclosure No. 7-212079)
Flattening of the powders of soft magnetic amorphous alloys, permalloy and molybdenum permalloy used in the know technologies requires long period for processing and productivity is low. Moreover, distortion by processing is large, and therefore, it is difficult to achieve high magnetic shield effect. Flake type sendust alloy has saturation magnetic flux constant of about 0.3.times.10.sup.-6 or less, but not zero, and therefore, when used as magnetic shield material, stress during flattening step or at use lowers the magnetic properties and thus magnetic shield effect as expected cannot be obtained.
Soft magnetic powder for magnetic shield of the above described Fe--Si--Cr alloy has high deformation resistance and require long period of time for flattening the powder. Due to insufficient corrosion resistance O.sub.2 -content of the powder increases during the flattening step as shown in FIG. 6, and the increase of O.sub.2 -content causes increase of coercive force at D.C. and decrease of permeability as shown in Fig, 7. This results in lowered effect to shield electromagnetic wave and magnetic field.
The above mentioned shield material prepared by coating insulating soft magnetic layer of a mixture of Fe--Al--Si alloy flat fine powder and an organic binder has a drawback that, due to difficulty in flattening the alloy, it is difficult to heighten effect to shield the electromagnetic wave.
The inventors proposed some kinds of shield material for electromagnetic wave having flexibility prepared by dispersing powder of soft magnetic material in a matrix of rubber or synthetic resin.
It is current demand that electromagnetic shield materials maintain shield effect at such a high frequency band as 1 GHz or higher, which has been necessitated by elevating clock speed of personal computers. Thus, choice of the alloy composition, powder shape and size of the soft magnetic material is important.
On the other hand, depending on the aspect of use, flame resistance is often required to the electromagnetic shield materials. In case where the material is used for ordinary electronic appliances such as personal computers, it is sufficient that the material meets V1 standard of flame resistance test, while in case home electric appliances it is requested that the material meets V0 standard, i.e., flame resistance of such extent that burning caused by a burner flame extinguishes when the flame is withdrawn. Conventional sheets for electromagnetic shield could not achieve the V0 standard even if flame resistant grade of chlorinated polyethylene prepared by incorporating a flame retarder such as antimony trioxide is used as the matrix material.